1. Field of the Invention
This invention relates to a solid model construction method and an apparatus thereof in which the shape of a solid model is inputted as two-dimensional coordinate data in the form of two or three orthographic views for conversion to three-dimensional coordinate data showing a stereographic structure of a solid model.
2. Description of the Prior Art
As proposed in Japanese Patent Laid-open Publication 62-202271 and the like, it is known to provide systems where structural data in the form of three orthographic views (e.g. as shown in FIG. 8) are converted to shape data in the solid model form (e.g. as shown in FIG. 9), and the shape of the solid model is displayed as is.
A typical prior art procedure for forming a solid model from the three orthographic views by an image processing unit is as follows.
(1) Construction of candidate vertices
From individual vertices on the three orthographic views, those which are possible to be vertices on a three-dimensional coordinate view are selected to construct three-dimensional candidate vertices.
(2) Construction of candidate edges
If a projection pattern of a definite line connecting given two candidate vertices appears as a vertex or a definite line in all of the given three orthographic views, and no other candidate vertex is present between the two candidate vertices, that line is held. as a candidate edge.
(3) Construction of candidate face
A face surrounded by candidate edges on a same plane and including no other candidate edge within is held as a candidate face.
(4) Construction of true surface model
Using knowledge of three-dimensional shapes, candidate vertices and candidate edges conflicting as elements of a solid model are removed. For example, from a geometrical knowledge that "all candidate vertices and candidate edges are boundaries of candidate faces," isolated points and pendant edges are removed.
(5) Construction of solid model
A set of candidate faces that can be boundaries of the solid model is searched for. Since the solid model has a limitation that "all edges are shared by only two faces, and the components do not intersect other than on boundaries," all sets that meet this condition are determined.
A problem in the construction of the solid model from the three orthographic views is the potential ambiguity of the three orthographic views. Since the three orthographic views are two-dimensional pictures, they cannot represent all of the intrinsic three-dimensional shapes. Therefore, for example, a plurality of solid models such as A, B, and C in FIGS. 9A, 9B and 9C may be generated for a set of three orthographic views shown in FIG. 8. The number of solid models generated due to the ambiguity exponentially increases. For example, when there are m ambiguities at a position and n ambiguities at another position, there are present a total of m.times.n ambiguities.
Heretofore, in the prior art, a plurality of solid models corresponding to three orthographic views are sequentially prepared, sequentially displayed as candidates to be selected on a solid model display screen, the final solid model is determined by a user specifying a desired candidate among the candidates on the display screen.
3. Problems to be Solved by the Invention
When different directions of placement are allowed, the stereographic models of shapes A, B, and C in FIGS. 9A, 9B and 9C have 3 cases for type A, 8 cases for type B, and 8 cases for type C, and a total of 19 solid models must be prepared, among which the desired solid model is selected. Thus, the prior method has been such problems that as the number of selected candidates to be selected increases, a long time is required for preparation of solid models, it takes a long time to find a candidate desired by the user, and conversational operation is not possible.
Furthermore, in view of operability, it is desirable that all solid models are held, and a desired candidate is selected among them, but to realize this, large amounts of data must be held, and a storage system with a large capacity becomes required.
Therefore, with a view to eliminate such prior art problems, it is an object the present invention to provide a solid model construction method and an apparatus which is able to simplify the selection processing, to reduce constructing time of the solid models and to hold relatively small in amounts of data.